In the field of material transfer from one point to another, it is often desired to assess the level of material in a vessel in order to determine when to initiate a control event. These control events could include turning on a fluid transfer pump, opening valves or drains, or adding a material to the container. For liquids, as a reservoir of fluid becomes too high, some means is often used to transfer the fluid from the high reservoir to another location such as either another reservoir or to discharge the fluid into the environment.
A number of techniques have been utilized in the past to accomplish this goal. Each of these prior techniques has its disadvantages. For some technologies, a sensor must be in direct contact with the material to be sensed and may suffer from corrosion, chemical reaction or physical wear of the sensor, resulting in premature failure. For other technologies, the complexity and cost of implementation may be barriers to a cost-effective product.
In previous fluid systems a mechanical float has often been used whereby the float would actuate a lever arm and an electric switch, or other form of electrical contact. The float, being of a lower density than the fluid, would ride with the top surface of the fluid as it rises and falls. A mechanical linkage between the float and the switch may suffer from mechanical wear. Additionally the float must be made from a substance that is not attacked by the fluid. In some applications where the fluid is not homogeneous, such as sewage applications, the float may become tangled or blocked by materials dispersed in the fluid.
In systems using an optical emitter and receiver to ‘see’ the material, the sensor may be sensitive to variations in the clarity of the material. Over time, if algae slime is allowed to grow in the fluid reservoir, the algae may block the transmission of the beam of light and give false indications. If powder residue from a loose solid material is allowed to build up on an optical sensor, the emitter and/or receiver can become partially blocked and also cause false indications. Likewise, with infrared emitters and receivers, the surface of the sensors must be cleaned occasionally to have accurate transmission.
Ultrasonic technology has been used to reflect back from the surface of a material and sense the distance from the sensor to the surface of the material. These sensors require relatively expensive circuitry and microprocessor control to determine the distance based on the time it takes for an ultrasonic pulse to be emitted, hit the surface of the material, and bounce back to the source. If the surface of the material is agitated (particularly in fluids), reflections of the wave can bounce off at angles and then off the walls of the reservoir introducing error in the received waveform. It is quite common for a reservoir or basin to have the inflow of fluid at a high enough rate to cause waves and agitation of the surface of the fluid.
Conductive probes of stainless steel or similar metal are also commonly used. These metal probes are set at a specific vertical level, and when they contact a conductive material such as impure water, the water forms a conductive path between the probes and activates some other part of a circuit. These metal probes may suffer from corrosive or chemical attack by the fluid being sensed. These metal probes can also acquire a build up of contaminants on the surface that adversely affects the measurement. Some fluids may either vary in their conductivity or are not conductive at all and cannot be sensed accurately.
Other systems rely on sensing the pressure in a compartment or under a flexible diaphragm that is in contact with the fluid to be sensed. The amount of pressure sensed gives an indication of the height of the fluid above the contact point. These systems are highly sensitive to environmental temperature since the temperature also drastically affects the pressure in the compartment with the sensor. The flexible diaphragm can also be chemically attacked by the fluid or simply become aged and crack from continuous mechanical flexing.